Industrial processes depend extensively on heterogeneous catalysts for chemical production and mitigation of environmental pollutants. These processes often rely on metal nanoparticles dispersed onto high surface area support materials to both maximize catalytically active surface area and for the most cost-effective use of expensive catalysts such as palladium, platinum, ruthenium, or rhodium. However, catalytic processes utilizing transition metal nanoparticles are often energy intensive, relying on high temperatures and pressures to maximize catalytic activity.
Light-driven chemical transformations may offer an attractive and ultimately sustainable alternative to traditional high-temperature catalytic reactions. Metallic plasmonic nanostructures may be useful for photoactive heterogeneous catalysts. Plasmonic nanoparticles uniquely couple electron density with electromagnetic radiation, leading to a collective oscillation of the conduction electrons in resonance with the frequency of incident light, known as a localized surface plasmon resonance (LSPR). These resonances lead to enhanced light absorption in an area much larger than the physical cross-section of the nanoparticle, and such optical antenna effects result in strongly enhanced electromagnetic fields near the nanoparticle surface. An LSPR can be damped through radiative reemission of a photon, or non-radiative Landau damping with the creation of energetic “hot” carriers: electrons above the Fermi energy of the metal and/or holes below the Fermi energy.
In this context, “hot” refers to carriers of an energy that is a significant fraction of the plasmon energy that would not be generated thermally at ambient temperature. Plasmonic metal nanoparticles have been shown to induce chemical transformations directly on their surfaces, through either phonon-driven or charge-carrier-driven mechanisms in Au, Ag, Cu, and, recently, Al nanoparticles. Although these “good” plasmonic metals may show initial promise for plasmon-induced photocatalytic chemistry, in general they have been shown to not be universally good catalytic materials.
In comparison, non-coinage transition metals have historical precedence as excellent catalysts, yet are generally considered poor plasmonic metals, because they suffer from large non-radiative damping, which results in broad spectral features and weak absorption across the visible region of the spectrum. Many catalytic transition metal nanoparticles (Pt, Pd, Rh, Ru, etc.) possess LSPRs in the UV, but this is disadvantageous for photocatalysis because of poor overlap with conventional laser sources or, alternatively, with the solar spectrum. One option to increase transition metal nanoparticles absorption properties is to increase the transition metal nanoparticle size, which would redshift the optical absorption, but it also increases cost and reduces surface area, and therefore catalytic activity.
This invention was made with support from the following Welch Foundation Grants: C-1220 and C-1222.